A turbofan gas turbine engine comprises a fan, which comprises a fan rotor and a number of circumferentially spaced radially outwardly extending fan blades secured to the fan rotor. The fan is surrounded by a fan casing, which defines a fan duct. The fan casing is also arranged to contain one or more of the fan blades in the unlikely event that a fan blade becomes detached from the fan rotor.
If a fan blade becomes detached from the fan rotor, due to impact from a large foreign body, e.g. a bird, the released fan blade strikes a main fan casing containment region. The fan blade generally progressively breaks up under a buckling action. The fan blade increases in strength from the tip to the root and at some position between the tip and the root the remaining portion of the fan blade, including the root, no longer buckles. The remaining portion of the fan blade has substantial mass and is accelerated to impact a rear fan containment region of the fan casing.
It is necessary to provide additional material to the rear fan containment region of the fan casing to contain the remaining portion of the fan blade. The additional material may be in the form of an increase in thickness, the provision of ribs, honeycomb liners etc, which dissipate the impact energy by plastic deformation of the material. However, these methods of protecting the rear fan containment region add weight to the turbofan gas turbine engine.